This invention relates to a process for treating whole tailings from a hot water extraction process plant with the object of separating the water, contained in the tailings, from the greatest part of the contained solids.
The hot water extraction process is now well described in the literature. The process is used commercially in two very large plants to extract bitumen from the tar sand deposit located in the Athabasca area of Alberta, Canada. In general, the process involves: conditioning mined tar sand in a rotating drum by mixing it with hot water, steam and some NaOH; flooding the slurry from the conditioning drum with additional hot water; temporarily retaining the flooded slurry in a thickener--like primary separation vessel to produce separate streams of bitumen froth, concentrated sand tailings and dilute middlings; treating the middlings stream in a sub-aerated secondary recovery flotation cell to produce a second froth stream and a dilute tailings stream; combining the froth streams, diluting them with naphtha and then centrifuging the diluted froth in two stages to produce reasonably clean bitumen and a separate tailings stream.
Some or all of the various tailings streams so produced are normally combined and have heretofore been pumped to a pond where they are retained. Typically this combined tailings stream comprises 0.49% bitumen, 44.87% water and 54.64% mineral solids. Of the solids, about 12% are fine matter i.e. -44.mu.. For purposes of this specification, this stream is hereinafter referred to as "whole tailings".
The problem of what to do with the whole tailings is one of the most serious ones facing this industry. It is necessary to clarify the water to a condition whereby it may be recycled through the extraction process. However, the rate of settling of the fines is inordinately slow. Since water used in the extraction process should not contain more than about 3% by weight fines, it has been found necessary to use gigantic settling ponds in order to provide sufficient retention time to permit the water to clarify to an acceptable level. By way of example, the larger of the two commercial plants, wich is designed to produce 125,000 B.O.P.D., has a proposed pond area of approximately 9 square miles. The ponds have to be surrounded by dikes which are expensive to construct. Furthermore, the ponds cover mineable tar sands and it has been estimated that to move their contents into the mined-out pits and thereby make these tar sands available for production would cost in the order of two billion dollars. From the foregoing, it will be appreciated that providing a feasible process for reducing the size of the ponds or eliminating them altogether is highly desirable.
Many different solutions to the problem have been proposed and are disclosed in patents. However to date none of them has had a significant effect on the commercial operation.
One process which at first glance may appear similar to the present invention is that disclosed in Canadian Pat. No. 892,548, issued to Hepp et al. Hepp teaches clarifying process water, i.e. settled pond water, by flocculating the fines with a known flocculating agent, such as lime, and then removing these flocculated solids from the stream by precoat vacuum filtration. The flocculated solids collect on the surface or in the interstices of the filter aid (which may be diatomaceous earth or the like) and will eventually blind the filter--thus the process requires shaving the blinded layer of filter aid to continuously expose a fresh surface for filtering. It is to be noted that the water being treated contains 0.1 to 1.5% bitumen and up to 20% mineral solids, of which 80-100% are fines; also the patent requires the use of a precoat filter medium on the filter.
Applicants gave consideration to applying the process of Pat. No. 892,548 to whole tailings but determined it was impractical, as practise of the technique would involve a large filtration area, a consequent large energy requirement, and the thin pre-coat filter cake would be unworkable with the feedstock, which is mainly coarse particles.
Experiments were subsequently carried out to determine if direct vacuum filtration of the whole tailings would work. These experiments were unsuccessful, as it was found that the coarse particles would naturally settle relatively quickly and would build up a thick porous filter cake--however the slower settling fines would gradually settle onto the surface of this cake and blind it, thereby effectively shutting off filtration.
Consideration was then given to the idea of flocculating the fines before filtering; however this suggestion was rejected, as it was felt the flocculant would mainly affect the fines to form relatively slow settling agglomerates which would still accumulate on the surface of the coarse particle filter cake and blind it.
Experiments were then conducted to investigate mechanically raking the surface of the filter cake to break up the blocking layer of fines. However, these experiments were also unsuccessful because it was not possible to produce a dry filter cake.